Back rest for a chair



FIG. 1 is a perspective view showing the top, left side and front of a back rest for a chair which forms my new design, the broken lines of other parts of the chair in this and other figures being for illustrative purposes only and forming no part of my design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a right side elevational view thereof; and,

FIG. 5 is a top plan view thereof. 

The ornamental design for a back rest for a chair, as shown and described. 